Measurement and analysis of the ${}^{241}\mathrm{Am}$($n$,$\gamma$) cross section with liquid scintillator detectors using time-of-flight spectroscopy at the n_TOF facility at CERN

The ${}^{241}\mathrm{Am}$($n$,$\gamma$) cross section has been measured at the n_TOF facility at CERN using deuterated benzene liquid scintillators, commonly known as C${}_6$D${}_6$ detectors, and time-of-flight spectrometry. The results in the resolved resonance range bring new constraints to evaluations below 150 eV, and the energy upper limit was extended from 150 to 320 eV with a total of 172 new resonances not present in current evaluations. The thermal capture cross section was found to be ${\sigma }_{th}=678\pm 68$ b, which is in good agreement with evaluations and most previous measurements. The capture cross section in the unresolved resonance region was extracted in the remaining energy range up to 150 keV, and found to be larger than current evaluations and previous measurements.

Figure 1

The ${}^{241}\mathrm{Am}$($n$,$\gamma$) cross section at 300 K for the first three resonances for different measurements and evaluations, all calculated from resonance parameters.

Figure 2

A schematic view of the ${}^{241}\mathrm{Am}$ sample used in the experiment.

Figure 3

The $\gamma$-ray spectrum of the ${}^{241}\mathrm{Am}$ measured in two energy ranges with a HPGe setup [46]. Some characteristic $\gamma$ rays from $\alpha$ decay or from $\alpha$ induced reactions are shown as well.

Figure 4

A schematic view of the detector and sample setup used for this experiment. The distance between the two deuterated benzene, C${}_6$D${}_6$, detectors was 6.1 cm.

Figure 5

The simulated detector responses and measured spectra from calibration sources. The resulting energy calibration is shown in the inset. The highlighted regions correspond to the intervals where the calibrations sources were fitted.

Figure 6

The deposited energy spectra for the Am sample with and without beam, and the gold sample, corresponding to all neutron energies with times of flight between ${10}^4$ and ${10}^8$ ns. The beam-off spectrum is normalized to acquisition time, and the beam-on is normalized to neutrons received relative to the Am sample spectrum.

Figure 7

The measured ${}^{241}\mathrm{Am}$ spectrum together with the various measured and calculated scaled background components. The count rate is expressed in a width-independent logarithmic equidistant binning.

Figure 8

The ${}^{197}\mathrm{Au}$($n$,$\gamma$) capture yield and the fit of the normalization with sammy, using fixed resonance parameters from JEFF-3.1.2 [31]. The $y$ axis is multiplied by $\sqrt{E\text{(eV)}}$ for a better view of the thermal region. The lower panel shows the residual of the fit, defined as the difference of the experimental yield and the fit, divided by the uncorrelated weighted uncertainty of the data points.

Figure 9

The ${}^{241}\mathrm{Am}$ capture yield with a 300-keV threshold is fitted up to the first three resonances to obtain the thermal cross section value. The $y$ axis is multiplied by $\sqrt{E\text{(}eV)}$ for convenience, to allow a better view of the thermal region. The lower panel shows the residual of the fit, defined as the difference of the experimental yield and the fit, divided by the uncorrelated weighted uncertainty of the data points.

Figure 10

Comparison of the thermal capture yield for different thresholds, normalized to the 4.9-eV Au resonance with the same threshold (in red), and normalized to the 0.3-eV ${}^{241}\mathrm{Am}$ resonance (in blue) which itself is normalized to Au with a threshold of 300 keV.

Figure 11

An overview of the experimental thermal total neutron capture cross section values as extracted from experimental data library EXFOR. The larger gray band indicates the average and standard deviation of the central values (698.5 and 72.7 b). The smaller gray band corresponds to the weighted mean and its uncertainty, excluding values without uncertainty (679.5 and 6.5 b).

Figure 12

The resolved resonance range analyzed in this work. The blue points are the experimental data, and the red line is the fit performed with sammy. The first panel has 500 bins per decade, whereas the following have 5000 bins per decade.

Figure 13

The cumulative Porter-Thomas distribution for the $s$-wave resonances fitted (solid curve) by Eq. (9) and extrapolated to zero threshold (dashed curve), both for the set of observed resonances in this work (blue) and the evaluated data from JEFF-3.1.2 (red). The obtained values for $D_0$ and $S_0$ depend on the choice of the threshold, here chosen such that 2/3 of the resonances are included in the fit.

Figure 14

The total signal and background components in the URR with and without the filters in the beam.

Figure 15

The calculated correction factor obtained with the code sesh [57] and the interpolation curve $f\left(E_n\right)$.

Figure 16

The final unresolved cross section compared to previous data and evaluations.